During these years, many companies desire to obtain information regarding customers' (or employees') emotions from conversation between the employees and the customers in order to estimate the customers' satisfaction levels and improve the quality of marketing. Human emotions often exhibit through voice, and the pitch (pitch frequency) of voice is one of important factors in identifying human emotions.
An example of the related art for estimating a pitch frequency will be described. FIG. 18 is a first diagram illustrating the example of the related art. As illustrated in FIG. 18, the example of the related art includes a frequency conversion unit 10, a correlation calculation unit 11, and an identification unit 12.
The frequency conversion unit 10 is a processing unit that calculates a frequency spectrum of an input speech sound by performing a Fourier transform on the input speech sound. The frequency conversion unit 10 outputs the frequency spectrum of the input speech sound to the correlation calculation unit 11. In the following description, a frequency spectrum of an input speech sound will be referred to as an “input spectrum”.
The correlation calculation unit 11 is a processing unit that calculates correlation values between cosine waves of various frequencies and an input spectrum. The correlation calculation unit 11 outputs information in which the frequencies of the cosine waves and the correlation values are associated with each other to the identification unit 12.
The identification unit 12 is a processing unit that outputs the frequency of a cosine wave associated with a largest correlation value as a pitch frequency.
FIG. 19 is a second diagram illustrating the example of the related art. In FIG. 19, an input spectrum 5a is output from the frequency conversion unit 10. A horizontal axis for the input spectrum 5a represents frequency, and a vertical axis represents the amplitude of the input spectrum 5a. 
Cosine waves 6a and 6b are part of cosine waves received by the correlation calculation unit 11. The cosine wave 6a has peaks thereof at a frequency f [Hz] and multiples of the frequency f [Hz] along the frequency axis. The cosine wave 6b has peaks thereof at a frequency 2f [Hz] and multiples of the frequency 2f [Hz].
The correlation calculation unit 11 calculates a correlation value of 0.95 between the input spectrum 5a and the cosine wave 6a. The correlation calculation unit 11 also calculates a correlation value of 0.40 between the input spectrum 5a and the cosine wave 6b. 
The identification unit 12 compares correlation values and identifies a largest correlation value. In the example illustrated in FIG. 19, the correlation value of 0.95 is the largest value, and the identification unit 12 outputs the frequency f [Hz] associated with the correlation value of 0.95 as a pitch frequency.
Examples of the related art include Japanese National Publication of International Patent Application No. 2002-516420 and Japanese National Publication of International Patent Application No. 2002-515609.